DE 104 875 teaches the roll out of flat or strip-shaped pieces of sheet metal such that parallel strips of different wall thickness are formed on the piece of sheet metal. Thereafter the piece of sheet metal is bent transverse to these strips and soldered to form a tube, whereby the tube has longitudinal sections of different wall thickness.
EP 0 788 849 A1 describes a similar method for manufacturing tubes having sections of different wall thickness in which regions of different wall thickness running parallel to one another are formed from initially flat sheet metal by specific partial reduction in wall thickness. In this case, the rolled sheet metal is cut to size, shaped into a tube and joined along the butting edges and welded.
DD 244 083 A1 describes a known method for the production of tubes and profiles having a variable inner diameter over the length. Herein defined portions of a metal strip are rolled, while other portions of the strip remain unrolled and thereby maintain their original wall thickness. The metal strip rolled that way is then formed to a tube or profile in a profiling apparatus.
The periodical change from rolled portions to unrolled portions limits the rolling speed and can weaken the strip at the regions of transition When the unrolled portions pass the rolling apparatus, there is no guiding of the metal strip. From this there is no exact measurement of length of the unrolled metal strip portions possible.